Carl Schwendeman, Calin Tarau, and William G. Anderson

Advanced Cooling Technologies, Inc., Lancaster, Pennsylvania

Peggy A. Cornell

Glenn Research Center, Cleveland, Ohio

Alkali Metal Backup Cooling for Stirling Systems—

Experimental Results

NASA/TM—2013-216591

October 2013

Paper 6733
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Carl Schwendeman, Calin Tarau, and William G. Anderson

Advanced Cooling Technologies, Inc., Lancaster, Pennsylvania

Peggy A. Cornell

Glenn Research Center, Cleveland, Ohio

Alkali Metal Backup Cooling for Stirling Systems—

Experimental Results

NASA/TM—2013-216591

October 2013

Paper 6733

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS-2013)

sponsored by the Aerospace Nuclear Society and Aerojet

Albuquerque, New Mexico, February 25–28, 2013

Prepared under Contract NNX11CA59C
Acknowledgments

This research was sponsored by NASA Glenn Research Center under Contract No. NNX11CA59C. Rodger Dyson was the

Contract Technical Monitor. We would like to thank Nicholas Schifer of NASA Glenn for operating the Stirling convertor and 

assisting with the testing. Tim Wagner was Advanced Cooling Technologies, Inc. (ACT) technician for the program and constructed

the Variable Conductance Heat Pipe (VCHP) prototype.

Available from

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Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2013-216591 1 

Alkali Metal Backup Cooling for Stirling Systems— 

Experimental Results 

Carl Schwendeman, Calin Tarau, and William G. Anderson 

Advanced Cooling Technologies, Inc. 

Lancaster, Pennsylvania 17601 

Peggy A. Cornell 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

In a Stirling Radioisotope Power System (RPS), heat must be continuously removed from the General 

Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable 

temperatures. The Stirling convertor normally provides this cooling. If the Stirling convertor stops in the 

current system, the insulation is designed to spoil, preventing damage to the GPHS at the cost of an early 

termination of the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) can be used to 

passively allow multiple stops and restarts of the Stirling convertor. In a previous NASA SBIR Program, 

Advanced Cooling Technologies, Inc. (ACT) developed a series of sodium VCHPs as backup cooling 

systems for Stirling RPS. The operation of these VCHPs was demonstrated using Stirling heater head 

simulators and GPHS simulators. In the most recent effort, a sodium VCHP with a stainless steel 

envelope was designed, fabricated and tested at NASA Glenn Research Center (GRC) with a Stirling 

convertor for two concepts; one for the Advanced Stirling Radioisotope Generator (ASRG) back up 

cooling system and one for the Long-lived Venus Lander thermal management system. The VCHP is 

designed to activate and remove heat from the stopped convertor at a 19 °C temperature increase from the 

nominal vapor temperature. The 19 °C temperature increase from nominal is low enough to avoid risking 

standard ASRG operation and spoiling of the Multi-Layer Insulation (MLI). In addition, the same backup 

cooling system can be applied to the Stirling convertor used for the refrigeration system of the Long-lived 

Venus Lander. The VCHP will allow the refrigeration system to: 1) rest during transit at a lower 

temperature than nominal; 2) pre-cool the modules to an even lower temperature before the entry in 

Venus atmosphere; 3) work at nominal temperature on Venus surface; 4) briefly stop multiple times on 

the Venus surface to allow scientific measurements. This paper presents the experimental results from 

integrating the VCHP with an operating Stirling convertor and describes the methodology used to achieve 

their successful combined operation. 

Introduction 

Advanced Stirling Radioisotope Generators (ASRG) are an attractive energy system for select space 

missions, and with the addition of a VCHP, they become even more versatile. The ASRG system is 

powered through thermal energy from decaying radioisotopes acting as General Purpose Heat Sources 

(GPHS). A Stirling engine converts the thermal energy to electrical energy and cools the GPHS (Ref. 1). 

The Stirling convertor must operate continuously to maintain acceptable temperatures of the GPHS and 

protect their cladding. The addition of alkali metal VCHPs allows the Stirling to cycle on and off during a 

mission and can be used as a backup cooling system. The benefits of being able to turn the Stirling off 

(VCHP on) are: reducing the Stirling fatigue, creep on the convertor heater head, and reducing vibrations 

for sensitive measurements. Heater head creep would be reduced by lowering the temperature of the 

system using the VCHP. The VCHP addition should also increase the efficiency of the Stirling by 

providing a uniform temperature across the heater head. 
NASA/TM—2013-216591 2 

A potential Stirling mission supplemented with a VCHP is the Long-lived Venus Lander, where a 

long duration mission is desired in the Venus environment. The VCHP will provide longer component 

life with the ability to turn off the Stirling during certain time periods of the mission. The Venus Lander 

VCHP will have four key functions for the mission: 1) allow the convertor to rest during transit at a lower 

temperature than the nominal one; 2) pre-cool the modules to an even lower temperature before entry into 

the Venus atmosphere; 3) work at nominal temperature in Venus environment; 4) allow multiple brief 

stoppages of the convertor on the Venus surface to allow scientific measurements. The four key functions 

for the Venus Lander and the ASRG backup cooling concept were demonstrated with a Stirling convertor 

provided by NASA Glenn. 

Nomenclature 

Tr Reservoir temperature 

Tv Vapor temperature 

Concept Introduction 

The conceptual VCHP for a Stirling convertor has four critical components; an evaporator, a first 

condenser, a second condenser and a Non-Condensable Gas (NCG) reservoir as shown in Figure 1. The 

VCHP is atypical because of the presence of two condensers. The working fluid is sodium and is shown 

in orange starting at the GPHS and ending at the front. The NCG is shown in tan starting at the front and 

ending in the reservoir. The VCHP evaporator interfaces with the GPHS modules and transfers thermal 

energy to one of the condensers. The first condenser is located at the Stirling’s heater head and is active 

only if the Stirling convertor is operating. The second condenser is a radiator between the heater head and 

the NCG reservoir that is active when the Stirling is off as shown in Figure 1. The NCG reservoir is located 

after the second condenser. The VCHP was tested for two scenarios: the ASRG backup cooling concept and 

the Venus Lander four-feature concept. The two tested concepts for the VCHP are described below. 

Figure 1.—VCHP interfaced with the GPHS and 

Stirling convertor. 
NASA/TM—2013-216591 3 

(a) (b) 

Figure 2.—ASRG backup cooling concept—front location for Stirling on (a) and off (b) (Ref. 2). 

ASRG Backup Cooling Concept 

The ASRG backup cooling concept is the simplest concept and involves turning the Stirling on and 

off at intervals of a given duration. The ASRG concept is shown in Figure 2, where the GPHS module 

interfaces directly with the Stirling heater head. The VCHP is shown in red and blue. The red depicts the 

hot sodium vapor and the blue represents the colder NCG. The VCHP wraps annularly around the heat 

collector attached to the Stirling heater head. The predicted location of the front is shown in Figure 2 for 

each stage. Heat is input to the Stirling when it is operational and the front is before the VCHP radiator 

(second condenser) as shown on the left in Figure 2. When the Stirling is turned off, the sodium vapor 

temperature will rise slightly and sweep the NCG out of the second condenser for utilization as a radiator. 

In the ASRG backup cooling concept, the cold side adapter flange (CSAF) would be used as the heat sink. 

The tested VCHP prototype radiates to ambient air for simplicity. The front will recede when the Stirling 

is turned back on and the vapor temperature will drop returning the system to the initial state. The Stirling 

will be cycled on and off to demonstrate repeatability and the recovery of the system parameters. 

Venus Lander Four-Feature Concept 

The Four-feature concept models the VCHP operating on a Stirling powered mission to Venus. The 

four features are described below: 

Feature 1. Transit. (Fig. 3(a)) The Stirling is off (rests) at lower temperature during transit to Venus to 

reduce creep on the system. In this stage, the VCHP radiator (second condenser) is active and the 

VCHP reservoir is cold from heat losses to space. Vapor temperature is represented as Tv1. 

Feature 2. Pre-cooling. (Fig. 3(b)) The Lander is shortly going to enter Venus’s atmosphere and precooling of the system is desired. The Stirling convertor is turned on and the system is cooled to the 

lowest temperature of the mission (Tv2). 

Feature 3. Normal Operation on Venus. (Fig. 3(c)) The Lander is in the Venus environment with the 

Stirling operating at its nominal temperature. The vapor temperature is represented as Tv3, the front is 

before the second condenser and the NCG reservoir is warmed by heat gains from the atmosphere. 

Feature 4. Stoppage on Venus. (Fig. 3(d)) The Lander is in the Venus environment with the Stirling 

off because of vibration sensitive measurements. The second condenser is active. At the end of this 

feature, the Stirling convertor is restarted and all the parameters return to the values that correspond to 

Feature 3. 
NASA/TM—2013-216591 4 

(a) (b) 

(c) (d) 

Figure 3.—Schematic of principle for the Venus Lander four-feature concept (a) Stirling OFF during transit to Venus 

with a cold reservoir. (b) Pre-cooling the GPHS modules before re-entry. (c) Stirling operating on Venus with a hot 

reservoir. (d) Stirling stopped on Venus (Ref. 3). 

Stirling Convertor 

The Stirling convertor used for this test was an Advanced Stirling Convertor (ASC-1HS #2) 

developed by Sunpower, Inc. under contract to NASA Glenn Research Center (GRC) as a technology 

development project. The 80We class ASC-1 HS #2 arrived at GRC in November 2007 and has 

accumulated over 6,500 hours of operation in GRC’s Stirling Research Lab. It is a hermetically sealed 

unit, with the exception of the helium fill tube, that can reach 850 °C with a high-strength cast superalloy 

247LC heater head that allows for higher hot-end temperatures. The convertor is shown in Figure 4 in the 

vertical (heater head up) test position. Figure 4 also illustrates the conceptual integration of the VCHP 

with the convertor. 

 
NASA/TM—2013-216591 5 

Figure 4.—VCHP integration with the 

Stirling convertor. 

The ASC heater head is covered by a brazed Nickel 201 cylinder known as the heat collector. 

Substantial effort for test preparation focused on obtaining an accurate fit between the outside diameter 

(OD) of the heat collector and the inside diameter (ID) of the VCHP. At GRC, the OD of the convertor’s 

heat collector was ground to 59.72 mm (0.000 mm, –0.025 mm) to accurately fit within the ID of the heat 

pipe. At ACT, the VCHP was machined to an ID of at least 59.79 mm (0.025 mm, –0.000 mm). Final 

measurements with calipers showed that both surfaces still displayed some slight high spots, however, after 

dry-run mating between the two parts, four shims of 0.076 mm (0.003 in.) thickness 90° were able to be fit 

within the clearance gap. Boron nitride paste was used to coat the OD of the ASC heat collector and the ID 

of the VCHP to allow for thermal transfer between the two surfaces and to prevent galling during VCHP 

removal. 

VCHP Prototype Design 

The VCHP design was dictated by the Stirling’s geometry and particularly by the OD and the height of 

the heat collector. An annular VCHP configuration that fits over the heat collector was selected to maximize 

heat transfer area. The exploded assembly and final assembly of the first prototype are shown in Figures 5 

and 6. The VCHP has two primary segments, the annular portion and the pipe portion. The annular portion 

of the VCHP fits completely over the Stirling’s heat collector. Extending out of the annular part of the 

VCHP is the radiator (second condenser) and the NCG reservoir. The extension was necessary to put the 

radiator and NCG reservoir outside of the insulated area for testing. The radiator is exposed to ambient air 

and the temperature of the reservoir is controlled for the experiment. The details of the VCHP operation are 

shown. 

 
NASA/TM—2013-216591 6 

Figure 5.—VCHP components before assembly. Figure 6.—Assembled VCHP. 

Figure 7.—VCHP connected to Stirling convertor. 

The removable VCHP test apparatus on the Stirling is shown in Figure 7. The VCHP is designed to 

transfer heat from a heat source either to the Stirling or to the environment depending on the operation of the 

Stirling convertor and the VCHP reservoir temperature. The prototype is heated through the base with 

cartridge heaters simulating a GPHS module as shown Figure 7. 

The VCHP working fluid evaporates on the top of the heater block absorbing the input heat. When 

the Stirling convertor is operating, the annular VCHP transfers heat to the heater head through the first 

condensing surface as shown in Figure 7. When the Stirling engine is not operating, the VCHP working 

fluid condenses at the VCHP radiator (second condensing surface) transferring the heat to ambient. The 

top of the heat collector is insulated from the GPHS simulator by an air gap to direct the incoming heat to 

the cylindrical heat transfer area. The top of the heat collector rests on a small rim inside of the VCHP. 

The inner wall of the VCHP was minimized to reduce the heat transfer down the wall and to reduce the 

thermal resistance from the sodium vapor to the heater collector. 
NASA/TM—2013-216591 7 

Experimental Configuration 

The testing setup is shown in Figure 8 with the VCHP and Stirling convertor assembled. The VCHP 

mounts and clamps onto the Stirling heater head after which the insulation and the Stirling’s insulation 

cover are installed. The test position is vertical with the heater head up to accommodate the fixture of the 

Stirling convertor. The final combined hardware (with insulation and thermocouples hooked up) is shown 

in Figure 9. 

The thermocouple (TC) map for the VCHP is shown in Figure 10. TCs 1 and 2 are probe 

thermocouples that are inserted into the GPHS simulator. TCs 3 to 6 read the annular vapor temperature 

and are spot welded on the cylindrical surface. TCs 7 to 19 are also spot welded surface TCs, four of 

which are located on the radiator (12 to 15). The large number of TCs on the tube and radiator section 

gives an accurate depiction of the front location. TC 20 is a probe that reads the NCG temperature in the 

reservoir. Other parameters that were monitored were the power into the cartridge heaters and the 

electrical power delivered from the Stirling convertor. 

The GPHS is simulated with six 0.5 in. diameter, 2.5 in. (64 mm) length, 200 W cartridge heaters that 

are inserted into the nickel top of the VCHP. The heaters are made by Watlow with an Incoloy case. The 

NCG reservoir is heated by a 2 in. (51 mm) clamp heater wired to a PID controller to maintain a constant 

temperature. 

Figure 8.—VCHP assembled on to the ASC-1 HS. Figure 9.—VCHP/ASC-1 HS with insulation. 

Figure 10.—VCHP—Thermocouple map. 

 
NASA/TM—2013-216591 8 

Modeling 

The theoretical results for the performance of the VCHP with the Stirling are presented. The 

modeling is based on flat front theory and the modeling results for the four-feature and ASRG backup 

cooling concepts are detailed. 

Four Feature Theoretical Results 

The four-feature concept modeling is based on the defined operating conditions on Venus with the 

Stirling convertor on (feature 3). The vapor temperature of the third feature (Tv3) is set to 840 C due to 

the 850 C limit of the Stirling heater head. With the vapor temperature of the third feature defined, the 

required NCG mass that allows the NCG to occupy the second condenser is solved for. The NCG mass, 

temperature and desired position of the front determine the vapor temperature for the other features. The 

front position is defined by the operation of the Stirling convertor. If the convertor is on, the front is 

located before the second condenser. If the convertor is off, the front is located after the second condenser 

(radiator in use). The operating temperatures for each of the four features are shown in Figure 11 as a 

function of the nominal operating temperature, Tv3. The temperature for each feature is determined by 

the intersection of the vertical dashed line at 840 C. The vapor temperature of the features in test 

sequential order is Tv1 (transit) 774 C, Tv2 (pre-cooling) 758 C, Tv3 (nominal) 840 C and Tv4 

(stoppage) 850 C. The coldest vapor temperature is during pre-cooling given that the Stirling will 

operate at a lower temperature before entry into the Venus atmosphere. The warmest predicted 

temperature is during the Stirling stoppage in the Venus environment at 850 C. The actual experiment 

was run at lower temperatures for a safety margin from the Stirling temperature limit; Tv3 was set at 

approximately 814 C. 

Figure 12 illustrates how the GPHS power is distributed for each feature throughout the experiment. 

The top dotted line is the power input into the system and the solid line is the power rejected by the 

VCHP radiator. The dashed line represents the power available for the Stirling convertor. The assumed 

losses to the environment are 68 W and account for the difference between the input and accepted power. 

The power can go to the Stirling convertor or be rejected to the environment by the VCHP radiator. The 

radiator is active for features 1, 2’ and 4 when the convertor is off. Feature 2’ is a transition from the precooling stage to the nominal operation on Venus. The iteration of features 3 and 4 is to demonstrate the 

ability of the VCHP to handle the thermal load when the Stirling is off and to show the ability of the 

system to recover in the Venus environment. 

Figure 11.—Feature temperatures as a function of Tv3. 
NASA/TM—2013-216591 9 

Figure 12.—Power distribution versus time. Figure 13.—Four-feature concept temperatures versus 

time. 

The temperature profile for the four-feature concept is shown in Figure 13. The top horizontal axis 

shows the corresponding feature number. The vapor temperature is shown as the top solid line, the 

Stirling heater head temperature is the middle dotted line and both use the left hand vertical axis. 

The reservoir temperature is shown as the bottom dashed line using the vertical right hand axis in 

Figure 13. The initial vapor temperature is low at 775 C and drops as the Stirling is used for pre-cooling. 

The Stirling is turned on at the start of feature 2, slowly dropping its temperature by the temperature 

difference between the vapor and the heat collector. The Stirling is turned off and reaches the temperature 

of the vapor during the entry stage. A parabolic temperature profile was assumed for entry. The vapor 

temperature increases during the entry stage because no heat rejection occurs. The reservoir temperature 

increases and keeps the NCG in the second condenser. The Stirling is turned on at the start of feature 3 

(nominal operation), lowering its temperature. The Stirling is turned off during feature 4, simulating a 

convertor stoppage in the Venus environment. Vapor temperature slightly increases to move the NCG out 

of the radiator while the heat collector temperature, in the absence of a heat flux, increases to that of the 

vapor. The on and off pattern is repeated to illustrate the potential that the VCHP offers for repeatability. 

The NCG reservoir temperature profile is fixed relative to the environment. 

ASRG Theoretical Results 

The ASRG theoretical predictions are not graphically presented as they are similar to the first and 

second features shown above within the four-feature concept modeling results. In the ASRG experiment, 

the testing sequences will start with Stirling on (feature 2) to simulate normal operation of the ASRG, 

followed by Stirling off (feature 1), to simulate stoppage, and again Stirling on (feature 2), to simulate a 

restart as well as recovery of the nominal parameters. The NCG reservoir temperature is held at 

approximately 100 C. The modeling results show that an 18 C vapor temperature increase is required to 

transition from the first condenser to the second. 
NASA/TM—2013-216591 10 

Experimental Results 

Heat Loss Determination and Convertor Calibration 

The testing of the VCHP integrated with the Advanced Stirling Convertor (ASC) provided by NASA 

GRC started with the determination of the system heat losses and Stirling convertor calibration 

parameters. Both heat losses and calibration parameters were determined only for the features where the 

convertor operates, features 2 and 3. Heat losses, however, were determined with the convertor off. The 

total power (electrical power from the main power supply) was incrementally increased until steady state 

of the desired vapor temperature was obtained for each feature (2 and 3). The vapor temperatures for 

features 2 and 3 were 762 and 810 C, respectively. The power needed to maintain the steady state vapor 

temperature for each of the two features was considered as the heat losses for that feature. The heat losses 

were approximately 219 W in feature 2 and approximately 250 W in feature 3. After each steady state 

maintained by a power equal to heat losses, the Stirling convertor was started while the total power was 

increased incrementally to obtain an electric power delivered by the convertor of approximately 80 W in 

feature 2. The corresponding total power was 433 W resulting in a net heat accepted by the convertor of 

approximately 214 W during feature 2 (transit) and 183 W during feature 3 (normal operation in Venus 

environment). The electrical power delivered by the convertor in feature 3 was approximately 72 W. In 

each feature, during calibration, the parameters of the convertor (piston amplitude and excitation voltage) 

were tuned to maintain the same VCHP vapor temperatures (set points) as during the heat loss 

determination. The determinations corresponding to feature 2 are also valid for the ASRG backup cooling 

concept testing since feature 2 can represent the ASRG during normal operation. 

ASRG Backup Cooling Concept 

The ASRG backup cooling experiment has the Stirling convertor operating at normal conditions then 

turns off the Stirling while still maintaining the full thermal power input to the Stirling. As previously 

mentioned, the VCHP in features 1 and 2 defined earlier for the Venus Lander’s four-feature concept are 

applicable to the ASRG backup cooling concept demonstration. The test was run starting with feature 2 

(Stirling on) followed by feature 1 (Stirling off) and again feature 2 to demonstrate the restart and 

recovery of the normal operating parameters. The temperatures and powers of interest during the 

experiment are shown in Figure 14. The VCHP temperatures shown are the GPHS simulator, vapor 

temperature and average radiator temperature. The VCHP reservoir temperature was not shown as it is 

constant at 101 °C. The Stirling temperatures shown are for the heater head and for the cold side adapter 

flange (CSAF). The power produced by the Stirling is shown and references the right hand axis. The 

system was cycled through the three stages mentioned above: stage 1 Stirling on (20 to 60 min.), stage 2 

Stirling off (60 to 150 min), and stage 3 Stirling on (150 to 370 min). The total power into the system was 

maintained near constant at 433 W. The results for the test focus on proving the ASRG backup cooling 

concept where the Stirling is able to shut down and restart at a later time being passively assisted by the 

VCHP. 

Stage 1 starts with the Stirling on (feature 2), shown from time 20 to 60 min. Steady state was 

achieved at approximately 50 min. The GPHS simulator was at 774 °C, the vapor at 761 °C, the heater 

head at 702 °C, the radiator at 130 °C and the CSAF at 89 °C. The low temperature of the radiator 

indicates that it is occupied by NCG and not being utilized as a condensing surface. The front is before 

the radiator and heat is being transferred to the Stirling. The relatively high temperature difference 

between the vapor and the heater head is due to thermal resistance at the VCHP - heater head interface. 

The power produced by the Stirling is approximately 80 W. The heat losses through the radiator are 

estimated to be 3-4 W based on its surface area (6200 mm2

) and temperature. 
NASA/TM—2013-216591 11 

Figure 14.—ASRG backup cooling concept testing—temperature profiles

The Stirling is off (feature 1) during the second stage shown from 60 to 150 min. The vapor 

temperature increased to 777 °C, the heater head temperature increased to 775 °C and the radiator starts 

being active. The slight rise in the vapor temperature increases the sodium vapor pressure and forces the 

NCG gas out of the radiator until equilibrium is reached. The radiator reached a steady state temperature 

of 642 °C. The temperature of the heater head increased approaching the vapor temperature as the heat 

flux through the VCHP-heater head interface decreased to zero. The CSAF temperature decreased as the 

Stirling stopped rejecting waste heat to the CSAF. 

The third stage (feature 2 repeat) started with turning the Stirling back on and lasted from 150 min to 

the end of the experiment (370 min). The heater head cooled rapidly as the thermal energy is being 

converted into electrical energy, slowly dropping the vapor temperature and condenser temperature back 

down to the initial values in stage 1. The only temperature that did not return to the initial value was the 

average radiator temperature as the test was stopped prior to reaching a true steady state. Previous results 

with a Stirling heater head simulator show that the radiator does return to the initial value if the stage has 

a sufficient duration. The radiator temperature decreases throughout the third stage as the front retracts 

out of the condenser. The decrease in the vapor temperature decreased the vapor pressure and the NCG 

was able to reoccupy the radiator. The small drop of the GPHS simulator, sodium vapor, and heater head 

temperatures at 240 min is due to human error where the heaters were momentarily unplugged. 

Figure 15 shows the power profile for the ASRG backup cooling concept testing that includes: total 

power into the system, power into the Stirling and power generated by the convertor. The total power is 

delivered by the cartridge heaters acting as the GPHS and was 433 W. The input power to the Stirling was 

calculated by subtracting the heat losses previously determined from the total power output. The energy 

conversion efficiency of the Stirling convertor can be estimated from the input and output power shown in 

Figure 15. The calculated efficiency is approximately 37 percent aligning well with the given efficiency 

of 38 percent (Ref. 4). 
NASA/TM—2013-216591 12 

Figure 15.—ASRG backup cooling concept testing—power profiles.

Venus Lander Four-Feature Concept 

The last part of the VCHP-Stirling testing was the Venus Lander four-feature concept demonstration. 

The experiment was run during a period of time (“mission” time) of approximately 500 min. It can be 

seen in Figure 16 that during the “mission” time all four features were tested. The durations of each 

feature are given below, in addition to the plot: 

1. From 100 to 166 min ...Feature 1—Transit 

2. From 166 to 260 min ...Feature 2—Pre-Cooling 

3. From 260 to 274 min ...Entry (Stirling off) 

4. From 274 to 360 min ...Feature 3—Normal operation in Venus environment 

5. From 360 to 420 min ...Feature 4—Stoppage in Venus environment 

6. From 420 to 500 min ...Feature 3—Restart and normal operation in Venus environment 

Similar to the previous concept demonstration, Figures 16 and 17 show time dependent distributions 

of the relevant temperatures and powers, respectively. Transit (feature 1) starts with the Stirling convertor 

off (simulating the status during transit), where the vapor temperature is 781 °C and the radiator rejects 

the heat load at approximately 626 °C. The convertor is turned on at 166 min to start pre-cooling 

(feature 2). During pre-cooling, the vapor temperature decreases from 781 to 761 C since the heat is now 

accepted by the convertor. The condenser temperature plummets as the sodium vapor pressure decreases 

allowing the NCG to occupy the radiator and stop the heat rejection. During the first two features, the 

reservoir is maintained at approximately 101 C. 

Entry into the Venus atmosphere is simulated from 260 to 74 min where, in addition to the system not 

rejecting heat produced by the radioisotopes, it gains ambient heat and increases its temperature in mock 

descent. The Stirling convertor is off during entry and the increasing temperature of the reservoir 

temporarily blocks the movement of the sodium to the radiator. The vapor temperature increases to 

827 C while the reservoir temperature increases initially to approximately 485 C. The reservoir 

temperature oscillation at 260 min is due to the heater controller having issues getting to steady state. 

Entry is ended by the starting of the Stirling convertor at 274 min. 
NASA/TM—2013-216591 13 

Figure 16.—Venus Lander four-feature concept testing—temperature profiles. 

Figure 17.—Four-feature concept power profile.

 
NASA/TM—2013-216591 14 

The nominal operation in the Venus environment (feature 3) is shown from 274 to 360 min, where the 

Stirling convertor is on. The vapor, Stirling heater head and reservoir temperatures are settling at nominal 

values of 814, 756, and 465 C, respectively. The stoppage on Venus (feature 4) starts at 360 min by 

turning off the Stirling convertor. Consequently, the vapor temperature increases to 839 C, activating the 

radiator that reaches a temperature of approximately 542 °C as it rejects the heat that was input. Feature 4 

was run for a long duration to obtain an approximate steady state value of the radiator. The radiator does 

not achieve the same temperature as during transit (feature 1) as the heat losses are higher for the whole 

system and consequently the power rejected during stoppage is lower. 

After the stoppage, the Stirling convertor is restarted and the system parameters return to values 

corresponding to normal operation in the Venus environment (feature 3). The radiator temperature 

decreases rapidly as the convertor now accepts heat and the front retracts. The sodium cools down slightly 

and the NCG forces the sodium out of the radiator. It can be observed that during the entire mission, the 

temperature drop between the vapor and the heater head when Stirling was on was around 65 C during 

pre-cooling (feature 2) and around 56 C during normal operation in the Venus environment (feature 3). 

This difference is due to the fact that during pre-cooling, heat losses are lower than during normal 

operation so the heat flux through the interface is higher. As a general conclusion, the system worked 

reasonably close to the predicted parameters confirming the concept, however, the stoppage (feature 4) 

could have been longer to ensure a steady state radiator temperature. 

Figure 17 shows separately the power profile for the four-feature concept test including: total power 

into the system, power into the Stirling and electrical power generated. Again, the input power into the 

Stirling convertor was calculated by subtracting the heat losses through insulation from the total power 

delivered by the heaters. As mentioned before, the heat losses were previously determined as 219 W 

during pre-cooling (vapor temperature of 762 °C) and approximately 250 W during normal operation in 

Venus environment (vapor temperature of 814 °C). 

Conclusion 

The VCHP was successfully tested with a high temperature Stirling convertor, all test objectives were 

achieved demonstrating proof-of-concept of ACT’s heat pipe. The joint NASA/ACT test marks the first 

successful operation of a hot-end VCHP with a Stirling convertor. The VCHP was able to successfully 

demonstrate both the ASRG back up cooling concept and the Venus Lander four-feature concept with a 

Stirling convertor at NASA Glenn Research Center. In the ASRG back up cooling concept, the VCHP 

allowed the Stirling to be shut off with a minimum rise in the heater head temperature (approximately 

16 C) and to be restarted to achieve the initial condition. The Venus Lander four-feature concept testing 

illustrated the ability of the VCHP to passively operate as predicted for each feature. Good agreement was 

found between the model and the experimental results with only small discrepancies. The feasibility of 

using an alkali metal VCHP as the backup or primary cooling system for a Stirling convertor has been 

proven experimentally. 

References 

1. J. Chan, J.G. Wood and J.G. Schreiber, “Development of Advanced Stirling Radioisotope Generator 

for Space,” STAIF, Albuquerque, 2007. 

2. C. Tarau and W.G. Anderson, “Sodium Variable Conductance Heat Pipe for Radioisotope Stirling 

Systems - Design and Experimental Results,” IECEC, Nashville, 2010. 

3. C. Tarau, W.G. Anderson and C.J. Peters, “Variable Conductance Heat Pipes for Long-Lived Venus 

Landers,” Journal of the British Interplanetary Society, vol. 63, no. 9/10, pp. 336-344, 2011. 

4. P.A. Cornell, E.J. Lewandowski, S.M. Oriti, and S.D. Wilson, “Stirling Convertor Extended 

Operation Testing and Data Analysis at Glenn Research Center,” IECEC, Denver, 2009. 
NASA/TM—2013-216591 15 

Appendix—Acronyms 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

CSAF Cold Side Adapter Flange 

GPHS General Purpose Heat Source 

GRC Glenn Research Center 

NCG Non Condensable Gas 

PID Proportional Integral Derivate 

MLI Multi-Layer Insulation 

RPS Radioisotope Power System 

TC Thermocouple 

VCHP Variable Conductance Heat Pipe